A gasket is a material or combination of materials clamped between two separable members or flanges of a mechanical joint. The gasket functions to effect a seal between the flanges and maintain the seal for an extended period of time. FIG. 1 is a cross -sectional view of a gasket 20 clamped between two flanges 24, 28 of a mechanical joint 32. The flanges 24, 28, are secured together with bolt 36. FIG. 1 also illustrates common forces that may affect the joint 32, such as bolt load X, hydrostatic end force Y, and blowout pressure Z. The gasket 20, in many applications, must be capable of sealing the mating surfaces 40, 44, and be impervious and resistant to the sealed media. Such gaskets 20 also must be able to withstand the application of elevated temperature and pressure in many applications. PVC and FRP piping are commonly used in corrosive applications, such as encountered in chemical plants. It will be appreciated that piping systems using these materials are somewhat fragile and require a gasket that will effect a seal at relatively low bolt loads so as not to crack or otherwise damage the flanges. The gasket must also be dimensionally stable so as to maintain a seal during a range of possible thermal changes in the process and have broad chemical compatibility.
Prior attempts to address the problems associated with gaskets for use in fragile joints have included, for example, envelope gaskets, rubber gaskets, rubber/polytetraflouroethylene (PTFE) gaskets, filled PTFE sheet, microcellular/porous PTFE sheet, and composite PTFE sheet. PTFE is commonly employed for gasketing in severe or corrosive chemical environments as it has a number of desirable properties for use as a gasketing material. For example, PTFE is inherently tough, chemically inert, has good tensile strength, and is stable over a broad range of temperatures. However, pure PTFE polymer is not highly compressible under low flange loads, and also is prone to creep, both of which may result in loss of sealing pressure. Envelope gaskets are a composite structure consisting of a PTFE envelope which is filled with a more compressible filler such as compressed fiber or felt. The PTFE envelope provides chemical resistance while deformability is provided by the filler material. However, PTFE envelopes are relatively thin (0.010 to 0.020 inch) and can develop pin holes during manufacture or while in service, thereby exposing the filler to incompatible corrosive media, which may result in loss of sealing pressure. Such envelope gaskets also have the least compressible component, i.e., the PTFE envelope which is not highly compressible or deformable under low flange loads, as outermost gasket surface.
Rubber gaskets are used routinely in plastic and FRP flanges because of their compressibility and resiliency, and their ability to seal at relatively low bolt loads. However, rubber gaskets have limited chemical and temperature resistance, and the proper compound must be specified for each application. Thus, multiple process streams that use the same piping are likely to require a time-consuming and somewhat costly change of gaskets. Rubber/PTFE gaskets incorporate a bonded PTFE envelope at the inner dimension of a rubber gasket. The envelope enhances the chemical resistance while the rubber substrate provides compressibility and deformability. Again however, the PTFE envelopes are thin (0.010 to 0.020 inch) and can develop pin holes during manufacture or while in service, thereby exposing the rubber substrate to incompatible corrosive media. Likewise, the PTFE envelope which is not highly compressible or deformable under low flange loads, is outermost in a rubber/PTFE gasket.
Filled PTFE sheets with good compressibility can be achieved by incorporating microballoons into the PTFE sheet material. Although PTFE sheet material offers the flexibility to be trimmed and modified by an end user, filled PTFE sheet material typically requires relatively high bolt loads to seal. Microcellular/porous PTFE sheets can be produced using a number of techniques, one of which involves adding a filler to the PTFE prior to trimming the sheet and then removing the filler after the sheet is formed. Thus, voids remain in the sheet material which give it a desired porosity (i.e., microcellular PTFE). Another method involves a particular sequence of extruding, stretching, and then heating to form a product known as porous PTFE. However, microcellular and porous PTFE are generally very soft and flexible and can be difficult to install in situations where limited flange separation is possible. Further, because microcellular and porous PTFE sheets are porous, a gasket cut from either must be fully compressed to close off the voids to prevent leakage through the gasket, and gaskets cut from these sheets typically require relatively high bolt loads to seal. In order to address the rigidity issues associated with microcellular/porous PTFE material, it has been proposed to laminate layers of the porous microcellular and/or porous PTFE sheets to a metal or full density PTFE substrate, but testing has shown that these materials likewise require relatively high bolt loads to seal.